The present invention relates to a fuel cell system for generating electric power by hydrogen and oxygen, and more specifically to a fuel cell system having a compressed air feeder system for supplying air to a fuel cell.
A conventional example of a compressed air feeder system for a fuel cell is described in U.S. Pat. No. 5,434,016. The fuel cell system disclosed in this official gazette compresses air taken in from the atmosphere and supplies it to the fuel cell. Since exhaust from the fuel cell body has a higher pressure than that of the atmosphere, a power is recovered as the exhaust is released into the atmosphere and the recovered power is reused to compress the air taken in.
Another conventional example is disclosed in JP-A-2001-93553. In the fuel cell system described in this official gazette, one of scrolls in a scroll fluid machine that are formed on both sides of an end plate is used for compressing air and the other for collecting power of exhaust from the fuel cell. A volume ratio between a compressor and an expander is determined and an inner pressure of an air chamber of the fuel cell is set close to a predetermined value based on the volume ratio to have a high energy efficiency.
An appropriate value of an air pressure to be supplied to the air chamber of the fuel cell depends on the kind and specifications of the fuel cell. When only the power generation efficiency of the fuel cell body is considered, it is preferable to use a high air supply pressure because the higher the air supply pressure, the higher the partial pressure of oxygen in the air chamber will become and therefore the more the reaction will be accelerated. This allows the equipment to be reduced in size, resulting in an advantage of a reduced on-board weight when the fuel cell system is mounted on a vehicle. However, increasing the air supply pressure increases an energy required to compress the air. It also requires the fuel cell equipment to have enough pressure withstandability at elevated pressures. This will lead to an increased weight of the fuel cell equipment, which is detrimental to a vehicle-mounted fuel cell system. The optimum air pressure is determined taking into consideration these conflicting conditions as well as the characteristics of the fuel cell body itself. Not only does the fuel cell have an optimum air supply pressure but there is also an optimum value for air flow rate. That is, unless oxygen to be used for reaction is supplied in sufficient amount, the reaction is retarded. On the other hand, supplying air in excess volume increases the amount of wasted oxygen not used for the reaction.
By the way, the condition of use of the fuel cell is not the same at all times. The environmental conditions, such as atmospheric pressure, temperature and humidity, vary according to season and time band and the demands for the output electric power of the fuel cell is also changing every minute. Particularly in a car-mounted fuel cell, when the vehicle moves from a low-altitude area to a high-altitude area, when it enters or exits a tunnel, when it is running up or down a slope, or when a rapid acceleration is required, the service condition changes greatly. When these service conditions change, the inner pressure of the air chamber also changes. At this time the optimum values of the pressure and flow rate of the supplied air also change for the reasons described above. The conventional technologies described above do not refer at all to the changes in the service condition nor do they make any provision for implementing a compressed air feeder system that can cope with a significant change in the service condition of the vehicle-mounted fuel cell.
Further, JP-A-60-160573 describes a turbo-compressor system for fuel cells that controls a turbine air flow by detecting a pressure in a compressor chamber. This fuel cell system employs a turbo-compressor, and therefore it is not appropriate to use this system on a fuel cell powered vehicle that is subject to abrupt load variations. The technology described in this official gazette is intended for a stationary system and thus effects the control based on the pressure in the pressure chamber. As a result, the distance between a pressure detection point and a control valve is long, taking a long time before the control stabilizes.